Earlier quoted context omitted.
You mean Ukraine was a part of Rus and Russian empires going back 1000 years. It goes way beyond the Soviet Union...
Yes there are ethnic ties between them. That doesn't mean they are identical, or that one should rule the other. Should Scotland rule Ireland (or vice versa) because they are both Gaelic peoples that were part of the British Empire?
Simply history. Ukraine was never a 'country' until briefly in the 20's then the 90's.
Ukrainians have been Rus, or Russians, or Ruthenians, etc... for most of their history. They were once a single people. That's just a fact.
The differences in identity are the fact that western Ukraine has at times belonged to Austria-Hungary or Poland. Or the fact the Cossacks carved out small states amidst foreign rule. And likewise Novgorod and Moscow had small states during Muslim domination which also contributed to a unique Russian identity.
It's not as simple as 'Ukraine was part of USSR for a generation.
My ancestors were Ukrainian, spoke Ukrainian, identified as Ukrainian but were from Austria-Hungary and officially were called 'Ruthenians'.
The same thing that separates Russia and Ukraine is what separates the Czech Republic and Slovakia. One people (ish), just different historical circumstances at times creating different identities and eventually states.
Ukraine should be its own country today. But not because of made up history, simply because today they want to.